The present invention relates to a method and a system for protecting an interconnection between ring networks.
Wave Division Multiplexing (WDM) is a technology allowing the transmission of a number of optical signals through an optical fibre using different separated light wavelengths. In this way the information carrying capacity may be increased significantly. The capacity depends on the number of used wavelength channels and their bandwidth. The signal at each wavelength travels through the fibre irrespectively of the other signals, so that each signal represents a discrete channel with large bandwidth.
A ring communication network is made up of nodes, which are connected in tandem in a ring by a unidirectional communication path, such as an optical fibre. A node receives transmissions from an upstream node. The return traffic is transmitted downstream to the first node.
A drawback of such a network is that a break in the ring or a failure of a node would prevent any node upstream of the break/failure to communicate with any node downstream of the break. A usual solution to this problem is to in some way provide a second spare communication path parallel to the first, but in the opposite direction, see U.S. Pat. Nos. 5,365,510, 5,179,548 and EP 677,936. If communication fails somewhere on the first communication path, then the traffic is directed back on the second communication path and the wished node will thus be reached from the other side.
Another solution is to send traffic on two communication paths in opposing directions, but with a segment of the ring inactivated for data traffic, see xe2x80x9cElectronic lettersxe2x80x9d, Dec. 5th 1996, Vol.32, No 25, p 2338-2339, B. S. Johansson, C. R. Batchellor and L. Egnell: xe2x80x9cFlexible bus: A self-restoring optical ADM ring architecturexe2x80x9d. In the case of a fault the segment is moved to the fault. It is however not described how to achieve this in practise.
If two ring networks shall communicate, there still exists a weak point, namely the node that interconnects the two ring networks. This problem is solved in an electrical version in U.S. Pat. No. 5,218,604, in that two ring networks are interconnected via two parallel serving nodes. A ring network consists, in this case, of two parallel communication paths, of which one carries traffic in the clockwise direction and the other carries the same traffic in the counterclockwise direction.
In the first ring network traffic from both communication paths are received by both of the serving nodes via a so called xe2x80x9cdrop-and-continuexe2x80x9d property. In each of the two serving nodes a selector selects from which communication path received signals will be retransmitted.
The two serving nodes then retransmit the received signals in a second ring network. Each serving node transmits away from the other serving node, with the result that the two communication paths in the second ring network carry the same traffic. The node to which the traffic is sent uses a selector to select from which communication path signals will be received.
A purpose with the present invention is to provide a protected interconnection between ring networks. A ring network will for short be called a xe2x80x9cringxe2x80x9d. The protection is done by using two parallel interconnecting nodes on a ring, which are coupled with two corresponding parallel interconnecting nodes on another ring and are called gateways. Signals entering a ring are routed across both gateways. A gateway may receive signals from both directions of the ring, but only transmits away from the neighbouring gateway.
So far the invention is similar to the invention in U.S. Pat. No. 5,218,604. A problem with the invention in U.S. Pat. No. 5,218,604 is that if an optical version is done, then expensive and not reliable optical selectors for each wavelength will have to be used. Another problem is that amplified spontaneous emission (ASE) is not stopped, which leads to saturation, higher noise level and oscillations.
The present invention solves the problem by using a different kind of nodes than the nodes in U.S. Pat. No. 5,218,604 and in particularly by each ring comprising an inactive segment. The inactive segment ensures that nodes on the ring only receive signals from one of the gateways. In the event of a fault the inactive segment moves so it encompasses the fault. Hence, operation is ensured.
If the inactive segment lies between the gateways then one of the gateways features a detection mechanism that detects the situation. If the situation occurs, said gateway suppresses transmission and just the other gateway transmits.
Other differences to U.S. Pat. No. 5,218,604 is that they receive and retransmit all traffic in each node, which is not done in the present invention. Also, they have the same traffic in both rings and select from which ring to receive, but in the present invention it is possible to receive from both rings at the same time, since it is not the same traffic in both rings.
Advantages with the present invention are that a protected interconnection between ring networks are provided in a simple, autonomous and not expensive way.
In U.S. Pat. No. 5,218,604 there is also a problem of not being able to place nodes between the gateways. This is solved in an embodiment of the present invention by xe2x80x9cdividingxe2x80x9d the ring in an upper part and a lower part. The different parts of the ring use different wavelengths. The inactive segment is situated either in the upper or the lower part.
One of the gateways transmit always both in the upper and the lower part of the ring, using different wavelengths in the different parts, as mentioned. The other gateway transmit only in the part where the inactive segment is situated. The other gateway knows in which part of the ring to transmit and not due to the detection mechanism described above.